Integrated inductors are widely used in microelectronics and particularly in RF integrated circuits (Radio Frequency Integrated Circuits—RFICs) and in microwave monolithic integrated circuits (MMICs). Integrated inductors are known as key devices in low noise amplifiers (LNAs). LNA is a special type of electronic amplifier used in communication systems to amplify very weak signals received by an antenna. They are also widely used in microwave systems such as GPS, due to low losses in the microwave frequency range. Typically, one or more integrated inductor devices are used to fabricate it. The operating frequency of an LNA is an important design specification and is influenced by its elements. For this reason, the inductors used in the LNAs need to be featured by a high quality factor Q, for the required inductance values L. The quality factor Q is the parameter of the inductor which characterizes its performance and is defined as the ratio of the energy of the magnetic field that can be stored in the inductor to the electric energy losses during its operation. Using the inductor parameters, the quality factor is defined as Q=Lω/R, where L and R is the total inductance and resistance of the inductor and ω is the frequency at which it is measured.
Furthermore, due to the constant demand for higher frequencies of operation regarding the electronic circuits, there is a struggle to find ways to increase the bandwidth of operation of the integrated inductors. Two are the main frequency ranges of interest in the design process. Firstly, the resonance frequency, where the inductor loses its inductive properties, and secondly, the frequency of the highest quality factor value.
Another type of inductors widely used in microelectronics is the category of printed inductors which are fabricated or printed on PCBs (Printed Circuit Boards). The material which they are fabricated on is usually silicon based. Other materials such as fabric may also be used. The difference between the two types of inductors merely lies in the fabrication process. The printed inductor is fabricated on any printed circuit board (PCB), while the integrated one on an integrated chip following the rules imposed by the specific process at each case. This results in that the shape of the inductor and the design steps are in both cases the same.
Up to now, the design of integrated inductors mainly consists of connecting in series two or more inductors of single or multi-turn which are designed in the different metal layers provided by each technology. Each layer consists of a continuous spiral metal track forming the inductor turns. Different metal layers are isolated from each other by oxide layers between them. The series combination of inductors is achieved by so-called vias provided by each technology, which are vertical metal lines connecting two adjacent metal layers. The two free ends of the continuous track, formed by the inductors connected in series, are the ports of the entire inductor. The inductor design almost always starts from the top metal layer, since most of the technologies provide a special metal for this purpose, while by this way a better isolation from the substrate is achieved. In the specific case of a one layer and 2.5 turn inductor, designed at one layer, a metal track at a different layer is used, to connect the port inside the inductor, with a point outside of it. The connection of the metal line with the two connecting points is achieved by use of vias. Such a line which connects two coplanar points in general, but resides in a different—usually the lower next—layer is called underpass. Two points are connected with underpass when the direct (coplanar) connection would short circuit other points in the chip.
The main design parameters of integrated inductors comprise the outer diameter of the inductor, the width of metal tack lines and the distance between them, and the number of turns and layers.
Several methods have been reported to increase either the quality factor or the main operating frequency values of interest regarding inductors. Most of them rely on either of unconventional materials such as substrates GaAS, or the post-processing of the chip such as etching, after its fabrication introducing additional steps in the production of the chip, possibly resulting in dramatically increasing the fabrication cost.
Other methods are based on exotic technologies, such as MEMS, increasing even more the cost of the final product.
Finally, still other methods based on the existing conventional technology, such as patterned ground Shields, do not provide significant improvement due to parasitic effects such as so-called eddy currents.
For this reason there is a need to increase the quality factor and the basic operating frequency values of integrated inductors by using conventional low cost technology without introducing additional steps that would lead to an increased cost of the final chip.